This invention relates to a sensor for a coin acceptor and has particular but not exclusive application to a multi-denomination coin acceptor.
Coin acceptors which discriminate between coins of different denominations are well known and one example is described in our GB-A-2 169 429. The acceptor includes a coin rundown path along which coins pass through a coin sensing station at which sensor coils perform a series of inductive tests on the coins in order to develop coin parameter signals which are indicative of the material and metallic content of the coin under test. The coin parameter signals are digitised so as to provide digital coin parameter data, which is then compared with stored coin data by means of a microcontroller to determine the acceptability or otherwise of the test coin. If the coin is found to be acceptable, the microcontroller operates an accept gate so that the coin is directed to an accept path. Otherwise, the accept gate remains inoperative and the coin is directed to a reject path.
The coin sensing station includes a number of different coils which may be energised at different frequencies, which form individual inductive couplings with the coin under test as it passes through the coin sensing station. Hitherto, the inductive sensor coils have been connected in parallel oscillatory circuits, in the feedback path of an amplifier which maintains the circuits in oscillation. The individual oscillatory circuits are connected in the feedback path of the amplifier sequentially by means of a multiplexer and successive samples of the amplitude deviation that occurs are digitised and fed to the microcontroller. A problem with this prior arrangement is that it takes a finite time for each sensor coil circuit to establish itself in a steady oscillatory condition when it is sequentially switched into the feedback path of the amplifier. This in turn limits the speed at which the multiplexer can scan through the various sensor coil outputs. Also, electrical noise can degrade the accuracy of the outputs of the sensor coils.
EP 0 704 825 discloses a coin validator which includes a coil in a series resonant circuit. One end of the coil is connected to ground, while the other end is connected to the inverting input of a differential amplifier via a capacitor.
The present invention seeks to provide a sensor for a coin validator which can be scanned at a much faster rate than hitherto, and that is less susceptible to the effects of noise.
In accordance with the invention there is provided a sensor for a coin acceptor, comprising an inductor for forming an inductive coupling with a coin to be tested, connected in series between first and second capacitors in a self oscillating circuit, and a detector to detect changes in oscillatory characteristics of the circuit as the coin passes the inductor.
The first and second capacitors may have substantially the same values.
It has been found in accordance with the invention that the series connected circuit can be brought into operation much more quickly than with prior parallel circuits used hitherto, with a higher resistance to the effects of noise.
The sensor according to the invention may include a plurality of self oscillating circuits and a multiplexer configuration to connect the circuits sequentially to the detector.
The sensor may include means for applying a predetermined bias to the or each self oscillating circuit at switch-on in order to reduce switch-on transients.
The detector may detect the amplitude and/or the frequency of the oscillatory characteristics of the circuit.